The present invention is directed to the synthesis of taxol and other tricyclic and tetracyclic taxanes and novel intermediates thereof.
The taxane family of terpenes, of which taxol is a member, has attracted considerable interest in both the biological and chemical arts. Taxol is a promising cancer chemotherapeutic agent with a broad spectrum of antileukemic and tumor-inhibiting activity. Taxol has the following structure: ##STR1## wherein Ac is acetyl.
The supply of taxol is presently being provided by the bark from Taxus brevifollia (Western Yew). However, taxol is found only in minute quantities in the bark of these slow growing evergreens. Consequently, chemists in recent years have expended their energies in trying to find a viable synthetic route for the preparation of taxol. To date, the results have not been entirely satisfactory.
A semi-synthetic approach to the preparation of taxol has been described by Greene, et al. in JACS 110, 5917 (1988), and involves the use of a congener of taxol, 10-deacetyl baccatin III which has the structure of formula II shown below: ##STR2## 10-deacetyl baccatin III is more readily available than taxol since it can be obtained from the needles of Taxus baccata. According to the method of Greene et al., 10-deacetyl baccatin III ("10-DAB") is converted to taxol by attachment of the C-10 acetyl group and by attachment of the C-13 .beta.-amido ester side chain through the esterification of the C-13 alcohol with a .beta.-amido carboxylic acid unit.
Denis et al. in U.S. Pat. No. 4,924,011 disclose another process for preparing derivatives of baccatin III or of 10-deacetylbaccatin III of general formula ##STR3## in which R' denotes hydrogen or acetyl. As reported, an acid of general formula: ##STR4## in which R.sub.1 is a hydroxy-protecting group, is condensed with a taxane derivative of general formula: ##STR5## in which R.sub.2 is an acetyl hydroxy-protecting group and R.sub.3 is a hydroxy-protecting group, and the protecting groups R.sub.1, R.sub.3 and, where appropriate, R.sub.2 are then replaced by hydrogen.
Other semisynthetic approaches for the preparation of taxol and for the preparation of other taxanes which possess tumor-inhibiting properties have been reported in recent years, but each of these approaches requires 10-DAB or baccatin III as a starting material. As such, the supply of taxol and other taxane derivatives remains dependent at least to some extent upon the collection of various parts of plants from the remote corners of the world and the extraction of 10-DAB and/or baccatin III therefrom.